This invention relates to transferring latent electrical images. Specifically, the present invention relates to a method and apparatus for transferring latent electrical images from a photosensitive image receptor to an insulating image receptor with a conductive back.
Methods for transferring latent electrical images utilize the known principle of charge transfer (TESI process), a discussion of which is set forth in the book by R. M. Schaffert "Electrophotography", John Wiley and Sons, New York, 1975. According to this principle, a latent electrical image is produced in a photosensitive image receptor, and a charge distribution corresponding with the image is then transferred onto a dielectric image receptor. In subsequent steps, the charged dielectric image receptor, which possesses a high insulation capacity, is then toned with toner powder or in an insulating liquid containing dispersed toner, at a polarity which is opposite to the charge of the dielectric image receptor, and subsequently fixed so as to obtain an electrophotographically produced copy of a copying master.
The above method can be applied with the aid of photosensitive image receptors and a conductive base, for instance an aluminum cylinder with a layer of photoconductive semiconductor material. A disadvantage of such photosensitive image receptors is that the life of the photoconductive layer is heavily affected by the inevitable mechanical stress and, especially where the photoconductive material is selenium, by the electrical discharge phenomena occurring during the transfer.
An example of this method is disclosed in German patent disclosure No. 31 44 669. To produce a charge image on the insulating image receptor, the photosensitive image receptor is successively subject to the following process steps: (1) The protective layer is charged to a voltage value U.sub.1 under complete exposure of the photoconductive layer; (2) the protective layer is discharged in the dark to a voltage value U.sub.2 ; (3) the photoconductive layer is image-exposed; (4) the insulating image receptor is placed on the protective layer and charge carriers are passed to the free side of the insulating image receptor until in the contact zone between the insulating image receptor and the photosensitive image receptor the potential has adjusted to the potential value of the base of the photoconductive layer; and (5) the insulating image receptor is finally separated from the photosensitive image receptor by introduction of ions at the point of separation. This method is rather expensive, since it requires two preparatory corona discharge steps and, additionally, ion-containing air must be blown in for separation of the image receptors, requiring an additional ion supply.
It is also known to provide a conductive base with a photoconductive layer on which a protective layer is applied which is transparent, insulating, and itself not photoconductive, which specifically protects the photoconductive layer from mechanical damage. Such a protective layer also makes it possible to also use selenium as the photoconductive layer, since the selenium is then protected from the inevitable microdischarges which otherwise would destroy it. A method for transferring a latent electrical image onto another image receptor with the aid of a photosensitive image receptor coated with a protective layer is disclosed in "Electrophotographic Characteristics of Overcoated Photoreceptors" by I. Chen, in "Photographic Science and Engineers", Vol. 22, No. 3, pp. 168-176. There, the protective layer is first charged to a specific voltage value U.sub.1, again under complete exposure. Next, through a simultaneously triggered second corona discharge, the image exposure of the image receptor is effected, whereafter the contact with the other image receptor, for instance a dielectric, insulating paper, can take place for transfer of the charge. According to an alternative embodiment of this prior method, the image exposure may take place between the first and second corona discharges. But at least two corona discharges are necessary in both cases for the application of the method.
It has been one objective of the present invention to provide an improved method for transferring a latent electrical image.
A further objective of the present invention has been to provide a maximally simple process for transferring a latent electrical image which can be realized at a high charge transfer.
A yet further objective of the present invention has been to provide a method for transferring a latent electrical image requiring only one corona discharge step.
An even further objective of the present invention has been to provide an apparatus for carrying out the improved method.